1. Technical Field of the Invention
The present invention relates to lead acid batteries for use as an electric source for automobiles and in other fields, and more particularly to a grid for use in lead acid batteries and a process for producing said grid.
2. Prior Art
Currently, there is an increasing tendency to use a lead-calcium alloy as the grid for lead acid batteries because of its excellent properties, i.e. a small loss in electrolytic solution after assemblage of battery and only a slight self-discharge during storage.
As compared with the lead-antimony alloy which has been used prior to lead-calcium alloy, however, the lead-calcium alloy is inferior in castability and mechanical strength after casting. For these reasons, it has hitherto been conventional to form a lead-calcium alloy into a grid by rolling its ingot into a sheet and then subjecting the sheet to an expanding treatment.
Although lead acid battery using a grid made of lead-calcium alloy is excellent in the lifetime in charge-discharge cycle (a light discharge not exceeding 5% of the capacity in an atmosphere having a temperature of 40.degree. C.), while its lifetime in the charge-discharge cycle in an atmosphere having a high temperature (exceeding 70.degree. C.) tends to decrease rapidly. Today, temperature in the engine room of automobiles tends to become high due to the increasing elaborateness of engine room itself and the stagnation in traffic, so that the high temperature durability of lead acid battery as an electric source placed in engine room is an important problem.
One object of the present invention is to provide a grid for use in lead acid batteries which solves the above-mentioned problems, and a process for producing said grid.
Another object of the present invention is to obtain a grid for use in lead acid batteries by a process which comprises superposing a sheet or a foil of a lead-silver alloy or a lead-silver-tin alloy on a sheet bar of a lead-calcium-tin alloy, said sheet or foil of a lead-silver or lead-silver-tin alloy having a thickness smaller than that of the sheet bar, followed by subjecting the superposed materials to a cold rolling to form an elementary sheet in which both the materials are integrated and then subjecting the elementary sheet to an expanding or punching process.
Yet, further object of the present invention is to provide a grid for use in lead acid batteries which is easy to produce, low in cost and excellent in cycle lifetime at high temperatures.